This invention relates to the dimensional inspection of parts.
The inspection of manufactured parts for dimensional accuracy is often performed by coordinate measuring machines. Such machines typically have a workpiece support and a traveling bridge which supports a workpiece-contacting probe. The probe is movable along three mutually perpendicular axes, and position sensing devices are utilized to determine the locations of the probe when it is in contact with the surface of the workpiece in terms of x, y, and z axes. These locations are recorded and they provide the basis for calculations which indicate the dimensions of the workpiece.
Measuring machines are customarily calibrated by using gauge blocks of known dimensions. The gauge block is inspected by the machine being calibrated and the inspection results are compared to the known dimensions of the gauge block. Subsequent measurements from the calibrated machine are adjusted accordingly.
Such calibrations are based on the assumptions that the dimensions of a gauge block do not change, and that the errors in a measuring machine are relatively uniform from one point to another throughout the measuring volume. The first of these assumptions is false if the temperature of the gauge block is different from the temperatures at which it was initially gaged, as will usually be the situation on a shop floor where there is no precise temperature control. The second assumption is false because a measuring machine, due to its individual vagaries, will produce errors with magnitudes which vary from one point to another throughout the measuring volume.
In the metrology field, it is generally accepted that a coordinate measuring machine must be located in a controlled environment in order to provide accurate measurements. Specially designed measuring laboratories are often constructed so that temperature, air purity, humidity, and other environmental elements are closely controlled to avoid conditions which would adversely affect the accuracy of any measurements.
Under current industrial practices, tolerances are stricter than they were in the past, and inventories are minimized due to just-in-time inventory practices. In view of these situations, there is a recognized need to place measuring machines on the shop floor. This would permit inspection of the parts soon after they are manufactured, and the manufacturing conditions can be adjusted quickly when necessary in order to improve the dimensional accuracy of subsequently manufactured parts. Heretofore, efforts to provide accurate measurements on the shop floor have focused on increasing the durability and accuracy of such machines in ways which attempt to overcome environmental problems.
As a practical matter, measuring laboratories cannot be located at workstations throughout a factory. A measuring machine in an uncontrolled environment cannot provide measurements which are dimensionally accurate. The sizes and shapes of parts being inspected change in various ways, and the measuring machine itself also changes. Thus, measuring machines remain in the laboratories, and production parts must travel from the shop to the measuring laboratory. This requires extra handling of the parts and inherent delays in detecting problems when they arise.
The present invention, aptly called "comparative metrology," makes it possible to obtain accurate measurements from measuring machines located throughout a factory or elsewhere outside an environmentally controlled measuring laboratory.
This invention is untraditional in the respect that it accepts and tolerates the fact that raw dimensional measurements made on the shop floor will be inaccurate. The invention relies on the fact that, despite their inaccuracies, such measurements taken on the shop floor are repeatable within an acceptable range of variations.
The rationale behind the present invention is that it is impossible to compensate for everything which may create measurement errors. Rather than trying to overcome environmental problems by modifying the machine or controlling the environment, a reference part of substantially the same size and shape as the eventual production part is inspected in the laboratory so that its dimensions are known as precisely as is possible in the laboratory. This reference part is then taken to another measuring machine which is on the shop floor where there is a less controlled environment to which the reference part and the production parts are exposed. The reference part is inspected by the shop floor machine and a production part is inspected by the shop floor machine. The measurement data is processed to determine, with accuracy comparable to laboratory accuracy, how close the dimensions of the production part are to the specified dimensions.